BECQUEREL for JuniorsPavel ZarubinBaldin & Veksler Laboratory of High Energies

The BECQUEREL Project (Beryllium (Boron) Clustering Quest in Relativistic Multifragmentation) (http://becquerel.jinr.ru)
is oriented toward emulsion expositions by light stable and radioactive nuclei with energy of the order of few GeV per nucleon in the JINR Nuclotron beams. Observations of the fragmentation of light relativistic nuclei open up new opportunities to explore highly excited nuclear states near multiple particle decay thresholds. Our interest in such states is motivated by their predicted properties as loosely bound systems with spatial spread significantly exceeding the fragment sizes. Natural components of such states are the lightest nuclei having no excited states below particle decay thresholds, i. e. deuterons, tritons, 3He, and 4He nuclei. For instance, intriguing conjecture is that N-alpha nuclei near the N-alpha particle decay threshold can constitute a loosely bound dilute gas forming a Bose condensate. Its major signature is multiple particle decay with a narrowed distribution of relative velocities. Search for such states on the nuclear scale is of undoubted interest since they can play a role of intermediate states ("waiting stations") for a stellar nuclear fusion due to dramatically reduced Coulomb repulsion.
A principal experimental task consists in provision of a complete spectroscopy of final fragments - observation of dissociation events, determination of various channel probabilities, and fragment identification and velocity measurement. Our approach is grounded on spectroscopy of relativistic fragments of incoming nuclei at longitudinal exposures of emulsion layers.
The track detection is performed in emulsion layers measuring 100-200-0.5 mm3. The layers are assembled in few cm thick stacks. The stacks are exposed to a beam in the longitudinal direction. They provide multiple track visualization over the total solid angle with spatial resolution of about 0.5 micron. Their sensitivity extends from slow fragments down to relativistic single charged particles.A mean range of light relativistic nuclei in emulsion is defined by the cross section of their inelastic interaction with emulsion nuclei. It varies from 14 cm for 6Li nuclei to 9 cm for 24Mg in a BR-2 type emulsion.

The advantages with respect to low energy researches are the following:
1. interactions reach a limiting fragmentation regime above a collision energy of 1A GeV and complex nuclear composition of emulsion doesn't affect isotopic composition of incoming nucleus fragments,
2. reactions take the shortest time, especially in cases of electromagnetic and diffractive dissociations,
3. projectile nucleus fragments are collimated mostly in a narrow forward cone limited by an angle 0.2/P_0, where P_0 is a primary nucleus momentum; this allows one to obtain 3D image of separated tracks in a single emulsion layer,
4. ionization losses of projectile fragments correspond to the minimum and practically don't distort measurements,
5. detection energy threshold for projectile fragments is absent,
6. a record resolution of emulsion (0.5 micron) provides a record angular resolution; the excitation scale of a fragmenting system in the projectile rest frame is of the order of few MeV per fragmenting nucleon; in the case of the projectile nucleus fragmentation perfect angular measurements play the determinative role in the estimation of the excitation energy scale while their momentum per nucleon may be taken the same as for an incoming nucleus,
7. reliable determination of charges of relativistic fragments in provided over a wide range, nuclear clustering manifests itself in the isotope composition of projectile nucleus fragments via measurement of a total momentum by multiple scattering technique one can identify hydrogen and helium isotopes;
8. selection of peripheral interactions with minimal energy transfers to an incoming nucleus; namely, these events present a major interest for studies of in-flight multiple particle decays.

Visual scanning is concentrated on the events with a total charge transfer of an incoming nucleus to secondary particles in a narrow fragmentation cone. Emulsion nucleus fragmentation and meson production become reduced or even suppressed in this way. Such events amount to a few percent of the total number of inelastic events. In practice, this approach allows one to accumulate statistics of a few tens of peripheral events, which is sufficient for a reliable determination of dominating dissociation channels. Thus, emulsions provide an excellent opportunity of a complete observation and study of light nucleus multifragmentation in flight. The Web site http://becquerel.jinr.ru contains a collection of characteristic images and movie recordings can be obtained by means of a microscope filming with a CCD camera. Please, find them in MOVIES section of on our web site http://becquerel.jinr.ru.
After some learning of a microscope usage you will have to become familiar with a microscopic navigation in emulsion media making search of nuclear stars. We shall welcome your contribution in a more systematic emulsion scanning along nuclear tracks and statistics accumulation of peripheral interactions of a specified type of relativistic nucleus. Thus, you will be intuitively introduced to the world of atomic nuclei in a way which is a most spectacular and evident. Who knows, may be you find something never seen before – novices sometimes more lucky than experienced individuals – and photo of your discovery will be introduced in our web collection. All necessary reading can be found on our web site http://becquerel.jinr.ru among PAPERS, PREPRINTS, REPRINTS, and BOOKS. How much to read depends exclusively on you: more – better….
To provide a quality we have to introduce a limit on a simultaneous participation – 2 persons at maximum. The work demands calm and strongly motivated individuals. You will be supervised by the Head of Sector Dr. Pavel Zarubin@lhe.jinr.ru and personally consulted by our young scientists Denis Artemenkov@lhe.jinr.ru and Tatiana Shchedrina@lhe.jinr.ru Our common phone number is 62004. Our photos as well other collaborators can be found among PHOTOS on our web site http://becquerel.jinr.ru. If something wrong with this address try http://becquerel.lhe.inr.ru